Restriction in Insert

I have table, where if data is inserted in two attempts, after that, noone can neither insert or update the data in the following table,e.g.
First am just inserting the following value in MyTable:
--------------------------------------
ID Student Course Marks
--------------------------------------
1 Andy ASM <NULL>
2 Bob ASM <NULL>
3 Andy OB <NULL>
4 Bob OB <NULL>
---------------------------------------
Here, you can see, Marks column left blank, and other column being filled up, on next attempt, marks will be inserted only once and if someone wants to update or delete it, the TRIGGER should restrict it.
How can this be done??

create an INSTEAD OF Update trigger and have your logic similar to below
if exists
( select 'x' from <table> where mark is null)
begin
print 'u dont have permision for this '
end
else
begin
< your update logic here. >
end